Optical corrections for conditions such as presbyopia and astigmatism are referenced to particular angular orientations about an optical viewing axis. Stacked focal segments for treating presbyopia have different powers spaced along a common vertical axis. Cylindrical optical corrections for treating astigmatism distinguish optical powers in two orthogonal directions oriented with respect to a common cylindrical axis. To achieve the desired optical corrections, the vertical axis of the presbyopia correction and cylindrical axis of the astigmatism correction must be angularly oriented as intended about the optical viewing axis. Contact lenses for making such corrections generally incorporate some type of ballasting to maintain a preferred angular orientation of the lenses about the optical viewing axis while mounted on wearers' eyes.
One or more sections of the lenses can be made thicker or thinner than other sections of the lenses to provide the ballast. For example, anterior surfaces can be decentered inferiorly with respect to posterior surfaces of the lenses to achieve a thickness variation that increases along a vertical meridian from the superior to the inferior portions of the lenses (i.e. the inferior portions are thicker than the superior portions).
So-called “toric contact lenses” include toric optical zones that correct for refractive abnormalities associated with astigmatism. The prescriptions of such lenses typically specify a second-order spherical correction, referred to as power, together with both a second-order cylindrical correction and a rotational angle specifying an angular offset of a cylindrical axis from an orientation axis of the ballast. Toric contact lens prescriptions typically specify the angular offset in 5-degree or 10-degree increments, ranging from 0 degrees to 180 degrees.
Multi-focal contact lenses can also require similar ballasting, where the different focusing sections of the lenses are stacked in a particular direction. For example, bifocal or progressively focusing contact lenses can have focusing powers that vary along a vertical meridian of the lenses between superior and inferior portions of the lenses and can require some form of ballasting to maintain the desired vertical orientation on wearers' eyes. A ballasting that progressively varies the thickness of the lenses along the same vertical meridian can be used for this purpose. The different focusing sections of the oriented lenses can include separate corrections for astigmatism.
The progressive variation in thickness along the vertical meridian of the lenses tends to exhibit a so-called “prism” effect through an optical zone, producing a slight shift in focus position. As an optical effect, the shift is small and, normally, can be readily accommodated by wearers' natural vision systems. However, in addition to the slight shift in focus position, the directed thickness variation also tends to produce higher-order aberrations, particularly vertical coma, which is a third-order effect. Such higher-order aberrations can reduce imaging performance, especially under lower light conditions. In general, the higher-order aberrations tend to increase in significance with pupil size.